In railway signalling technology there are special safety requirements for the devices used for controlling, for example, switches and power to lamps in light signals. These devices must be made such that they do not unintentionally feed current to the load when a fault occurs in them. For a long time now these safety devices have been designed to contain and use a special type of relay, a so-called fail-safe signal relay. This also applies to the interlocking logic controlling the safety devices. More lately, the interlocking logic has been implemented with computer control of the fail-safe type. The safety devices themselves have still been made with relays however. From the Swedish Pat. No. 7801810-8, for example, there is known a safety device in which there are two command receivers for receiving a command with the same meaning from a central control apparatus and for combined switching of a device controlling railway traffic and connected to the safety device, these command receivers containing relay means for achieving the switching.